The present invention relates to a novel composite material and uses therefor. More particularly, the invention relates to a composite material which may be used for producing a printed wiring board having fine wiring patterns.
With the recent miniaturization and densification of electronic equipment, the pattern widths and spacing of printed writing boards used for electronic equipment have become smaller year by year. As a result, the copper foil used for forming wiring has tended to become thinner, for example, from 35 xcexcm or 18 xcexcm to 12 xcexcm.
When using such metal foils, a printed wiring board may be produced by the steps shown in FIGS. 1(a)-(f) (a panel plating process). A metal foil 2 is bonded to a substrate 1 made of an insulating resin to form a laminate as shown in FIG. 1(a). In order to electrically connect the lower wiring to the upper wiring, a hole is opened by drilling or with a laser beam to form a via hole 3 as shown in FIG. 1(b). Then, electroless plating and electroplating are used to provide a copper layer 4 on the metal foils 2 and the via hole 3 as shown in FIG. 1(c). The surface of layer 4 is coated with a resist 5 which is irradiated through a photo mask and developed, thus defining the wiring pattern as shown in FIG. 1(d). Thereafter, the plated layer 4 and the metal foil 2 are removed by etching to leave the desired wiring pattern 6 as shown in FIG. 1(e), after which the resist is removed as shown in FIG. 1(f).
In the production of printed wiring board by the panel plating process, a wiring having a finer pitch can be formed by reducing the thickness of the metal foil. Thus, the need for thinner metal foils has increased. It is difficult to handle an extremely thin metal foil, and tears or wrinkles sometimes occur when the metal foil is bonded to the insulating substrate to form a laminate. Further, in the panel plating process, when the laminate is directly irradiated with a laser beam to form a via hole, a burr may be formed at the edge of the metal foil near the via hole. Because plating on such burrs proceeds more rapidly than plating on metal foil, the burrs become larger, causing problems with adherence of the plating on the metal foil and of the resist. For this reason, the metal foil at the hole opening must be removed by mechanical abrasion or the like, adding complexity to the process of making via holes.
There is also a process for directly producing a printed wiring board without using any metal foil. In this process, a printed wiring board is prepared through the steps shown in FIGS. 2(a)-(e) (a pattern plating process). A substrate 11 made of an insulating resin has a hole opened by drilling or with a laser beam to form a via hole 12 as shown in FIGS. 2(a) and 2(b). Then, the substrate is coated with a resist 13 and irradiated through a photo mask and developed to create the wiring pattern as shown in FIG. 2(c). Electroless plating and electroplating are using to form a plated layer as shown in FIG. 2(d), after which the resist 13 is removed, leaving the desired wiring pattern 14, as shown in FIG. 2(e).
The pattern plating process wherein metal foil is not used is advantageous in that it is feasible to make a thin surface plated layer and the steps of the process are simple. It is necessary, however, to roughen the surface of the insulating resin substrate by chemical or physical means to enhance bond strength between the resin and the conductive material. Even if the surface is roughened, however, insufficient bond strength between the wiring pattern which is formed and the insulating resin is a problem. Also, the printed wiring board obtained by the pattern plating process does not always have satisfactory heat resistance. Therefore, when the printed wiring board is heated, such as when soldering is used to mount electronic parts, the bond strength between the wiring pattern and the substrate is weakened, and the wiring pattern may separate. Moreover, since the metallic layer formed is brittle, breakage of the wires is occasionally observed because of applied flexure stress.
The present inventors have found that use of a composite material comprising a carrier having a releasable conductive fine particles on its surface makes it possible to produce a printed wiring board free from all of the above problems and permits formation of wiring patterns having very fine pitch.
The present invention is intended to solve the problems discussed above, and to provide a composite material capable of producing printed wiring boards having an excellent bond between the wiring pattern and the substrate and excellent heat resistance, with very fine wiring lines and pitch.
The composite material according to the invention comprises a carrier having releasable conductive fine particles on its surface.
The conductive fine particles have dimensions of 0.1-5.0 xcexcm relative to the thickness of the composite material.
In the composite material, the surface roughness (Rz) of the carrier on which the conductive fine particles is formed preferably is in the range of 1.0-10.0 xcexcm.
The invention also includes both a laminate made by laminating the composite material described above onto a surface of a substrate, and the resulting laminate from which the carrier has been removed.
The printed wiring board according to the invention is made by using the composite material described above.